Known infrared detecting devices include an infrared detecting element and a signal processing board (refer to Patent Literature 1, for example). The infrared detecting element includes a semiconductor substrate in which a plurality of pixels are two-dimensionally arranged. A signal processing board includes a plurality of signal processing circuits configured to process signals output from the plurality of pixels. The signal processing board is disposed to oppose the semiconductor substrate. The plurality of signal processing circuits are placed in an area opposing the semiconductor substrate.